Elevators, flour silos, flour mills and bakeries are classified as hazardous due to the presence of an explosive and flammable environment. And flour dust is to blame. It is generated during transport and transfer of flour, forming mixtures with air (aerosols) or accumulations on surfaces (aerogels). Let's find out why flour explodes and how to protect production from such accidents.
Some statistics on incidents
Every year, 400-500 accidents occur in storage and processing facilities around the world. Of these, more than half occur in grain elevators and flour mills. Here are examples of typical flour explosions
- 17 July 2015 — Bosley mill explosion in Congleton, Cheshire, UK. Four people were killed; a four-storey building was destroyed. Source of the accident: wood flour explosion.
- 5 November 2015— Explosion at a bakery in Brasov, Romania. 6 people were injured, including 1 worker with multiple fractures and burns. Source of the accident: explosion of flour dust in the ventilation system.
- January 12, 202 — Dust explosion at a feed mill in the village of Belenkoye, Borisovsky District, Belgorod Region, Russia. Five people were injured; there was a partial collapse of structures and a fire.
- September 14, 2021 — Explosion at a bakery factory in Savannah, Georgia, USA. Two people were injured; $25,000 in damage. Source of the accident: explosion of flour dust in the flour mixing room.
The reasons for explosions in flour mills are mostly the same:
- non-compliance with dust control, i.e. untimely removal of organic dust;
- Failure to comply with fire safety regulations;
- Non-trained personnel allowed to work;
- Operation of obsolete equipment or non-compliance with operating rules;
- Failure to comply with the rules for handling plant raw materials;
- poor management of operations at the site.
Owners and engineers of plant material processing facilities must remember: flour explodes and burns. This is due to its properties
Flour explosiveness
In the standards for explosion and fire safety there is a term such as " The Lower Explosive Limit (LEL)". This is measured in grams per cubic metre and indicates at what concentration of dust in the air a fire is possible. If it is lower, no ignition will occur.
All air and dust media where the LEL is below 65 g/m3 are classified as explosive. An ignition in them can result in a local primary explosion which can lead to a series of devastating secondary explosions.
Flour is explosive precisely because of its LEL. It amounts to:
- for wood flour 11.2 g/m3;
- for flours (rye, barley and other cereals) 20-63 g/m3
- beet sugar: 8.9 g/m3;
- fine flour dust - 10.1 g/m3.
These figures show how many grams of organic dust of a certain type must be contained in a cubic metre of air before an explosion occurs in contact with an ignition source (electric spark, heated surface or lit match).
Flour dust in the form of aerosols is always dangerous. Moreover, the higher the sugar content, the greater the risk – see LEL examples. Fine dusts also pose a particular risk, as they settle more slowly. Each particle reacts with oxygen in the air, which is an oxidising agent and supports combustion.
Flour in the form of aerogels, i.e. settled on the surface, is less dangerous. But it must not be allowed to accumulate. Firstly, it is capable of self-ignition - wood flour catches fire at 255°C. Secondly, if there are air turbulences, e.g. due to ventilation, wood aerogels will rise into the air and become explosive aerosols.
Flour dusts are class 1 explosive as flammable with rapid spreading of the explosion flame. A concentration of 10.1 g/m3 and the heat of a burning match is sufficient to ignite, and at a concentration of 28 g/m3 the explosion pressure will be maximum.
Causes and locations of flour dust
Organic dust is a by-product of many industries. In food processing plants it is generated:
- during grain processing;
- during transport of raw materials;
- during loading, blending and packaging of flour.
Flour dust accumulates in bunkers, flour silos and bucket elevators. It is present on packaging lines, in flour milling machines and on conveyors.
Especially suction systems that are not maintained in a timely manner pose a hazard. The filters in suction systems get clogged, and concentrations of flour dust in the suction systems are high. If the ventilation is activated, this dust will be blown into the air and a local explosion may occur.
The international standard IEC 60079-10-1:2020 classifies explosive atmospheres in Zone 20. This is a place where an explosive atmosphere in the form of a cloud of combustible dust, fibres or volatile particles in the air is present continuously or for long periods or where layers of dust of arbitrary or excessive thickness are formed.
Zone 20 includes:
- cyclones and filters;
- mills;
- feed hoppers;
- silos;
- dust product conveying equipment, except for parts of conveyor belts and chains;
- mixers, driers, containers.
Dust explosions cannot be completely eliminated in food processing plants. However, it is possible to reduce the likelihood of an explosion occurring and localisation measures can be taken to protect people and equipment.
Explosion prevention and explosion protection in the flour industry
Two types of measures must be taken in all plants with explosion and fire hazards:
- Accident prevention measures
- Localizing accidents and removing their consequences.
Examples of such measures for protection against an explosion in a flour mill can be found in the table below.
Protection
- Carry out an explosion safety audit of the project organisation. Classify hazardous areas and develop measures to protect them
- Prevent destruction of equipment use explosion relief devices - explosion dischargers. Most often installed on mills and crushers where it is possible to vent the explosion energy into the open space.
- Use flame arresters and explosion containment systems to protect people and equipment in enclosed spaces. For example HRD barriers.
- Use alarms and electrical shutdown devices that stop equipment operation in case of abnormal situations.
- Use fire suppression systems.
- Ensure periodic training of personnel to practice actions in case of accident and fire.
Ex-protection and explosion protection measures are developed individually for each hazardous location. Legislation contains general requirements which must be adapted to the operating conditions in the individual company.
This work is carried out by specialist organisations. They can, for example, be commissioned to calculate the explosion protection panels for flour storage bins. Designers determine the class of devices required, the location of their installation taking into account the specifics of the individual facility. ATEX.CENTER’s specialists can carry out all design work and select the technical solution on a turnkey basis.